Magnetic resonance tomography (MR or MRT) is an imaging method for displaying tissue in the human or animal body. MRT is based on the principle of nuclear spin resonance, in accordance with which atomic nuclei such as the hydrogen nuclei present in large numbers in the body exhibit a magnetic moment. They can thereby be excited with electromagnetic radiation in the radio frequency region (RF radiation) in an applied external magnetic field, and emit this radiation shortly thereafter. This RF radiation is detected with an antenna that mostly also generates the exciter pulse; this is why use is made of the term RF transceiver system, or RF coil, for short.
The magnetic field is mostly generated by a superconducting main magnet that is integrated in a field generating unit that encloses a horizontal patient tunnel into which the patient to be examined is pushed. The main magnetic field then runs parallel to the longitudinal direction of the tubes, in the so called z direction.
The resonant frequency of the atomic nuclei is directly proportional to the applied main magnetic field. Consequently, the spatial coding inside an image volume is achieved by virtue of the fact that so called gradient fields are applied in addition to the main magnetic field during the measurement; these are briefly applied magnetic fields with as linear as possible a gradient in the x, y or z directions. The gradient fields are mostly generated by specific gradient coils that are arranged inside the field generating unit.
A further medical imaging method is positron emission tomography (PET). As a nuclear medicine method, PET is suitable, in particular, for displaying biochemical processes in the body, for example for finding tumors and metastases. In this case, the patient is administered a tracer with a radionuclide that is distributed in the body and emits radioactive radiation in the form of positrons in the process. After a short time, the positrons decay into two opposite gamma quanta that are captured by suitable detectors. These are mostly arranged around the body as an annular PET detector. For example, the photons are captured by a matrix, made from scintillation crystals, in which each photon produces a scintillation upon striking. Said scintillation is, in turn, captured and amplified by photodetectors, for example by photomultiplier tubes or avalanche photodiodes.
Interest has recently been taken in combining MRT and PET with one another in one device in order to be able to apply the two imaging modalities simultaneously or shortly after one another to the same patient. This requires arranging the two units of MR-RF transceiver system and PET detector required for data acquisition inside the (mostly superconducting) main magnet and MR gradient coil.
In an obvious solution, to this end a PET detector is inserted into the patient tunnel of the field generating unit of an MR device, and, in turn, an RF transceiver unit is inserted into the PET detector ring. However, this arrangement is problematic, since the RF coil and PET detector exert a negative mutual influence: the currents inside the PET detector generate interference fields that are captured by the RF coil and can lead to interference signals. Since it is arranged between the examination region and the PET detector, the RF coil, in turn, can lead to scattering of gamma quanta and thus reduce the sensitivity of the PET detector. Moreover, nesting the RF coil and PET detector ring from the inside to the outside strongly reduces the inside diameter remaining for the patient inside the main magnet, and this can, in particular, result in the measurement being incapable of being carried out given a patient who is claustrophobically disposed or overweight.
An example of an MR device in which a PET detector is arranged between the gradient coil and the RF transceiver system is illustrated in FIG. 2. FIG. 2 shows a longitudinal half of the field generating unit 9 of an MR device, with further components integrated therein, in cross section. The dashed and dotted line 2 represents the middle line of the substantially tubular field generating unit 9. Permanently integrated in the field generating unit 9 is a gradient coil 3 that is likewise approximately tubular and that has coils for generating x, y and z gradients.
Inserted into the gradient coil 3 is a PET detector ring 4, an RF shield 6, a support tube 5 and an RF transceiver system 7. The RF shield 6 ensures that the RF fields are shielded against the PET ring during excitation of the RF coil 7. The RF coil 7 is provided with a cladding 8 against the examination region and/or patient tunnel 13.
The design illustrated in FIG. 2 therefore has the advantage that a whole body examination is thereby possible. The designation “body coil” is also used for an RF coil 7, integrated permanently in the main magnet 9, according to FIG. 2, which permits excitation and detection over the entire examination region.
On the other hand, the arrangement illustrated in FIG. 2 has the abovementioned disadvantages, for example it diminishes the onion skin type design of the patient tunnel. In order still to enable a sufficiently large inside diameter, the distance between the RF shield 6 and the RF conductor structures that is required for the formation of the field return space and thus for a good quality of the RF coil 7 must be strongly reduced inside the coil 7.